Why is one of my objects is not visible in the final rendering?
you can check if relevant objects are enabled/disabled in render.
I usually prefer to show it in outliner like this:


click on filter icon and activate the button with camera icon





then you can see it in outliner and check if object is enabled/disabled for rendering


 

and since you are using modifiers you should check if render is enabled for modifier too (it is always the camera icon)